Afterburner system



Aug. 24, 1965 T. A. BADEN AFTERBURNER SYSTEM 3 Sheets-Sheet 1 Filed 001;. 29, 1962 III M INVENTOR. THoMAs A. BADEN Aug. 24, 1965 1-. A. BADEN AFTERBURNER SYSTEM Filed Oct. 29, 1 962 3 Sheets-Sheet 3 United States Patent 3,201,933 AFTEURNER SYSTEM Thomas A. Baden, Minneapolis, Minn, assignor to Donaldson Company, Inc., Minneapolis, Minn, a corporation of Minnesota Filed 'Oct. 29, 1962, Ser. No. 233,736 1 Claim. (Cl. 60--30) This invention relates to exhaust afterburner systems. I More particularly, this invention relates to regenerative exhaust afterburners and to manifold means therefor. Still more particularly, this invention is directed to a flame type, flow reversal, regenerative exhaust afterburner and to an improved exhaust gas input manifold assembly therefor. I It is an object of this invention to provide an afterburner for internal combustion engines which combusts exhausted unburned or partially burned fuel.

It is another object of this invention to provide an afterburner system which is similar in back pressure to a conventional mufiier system.

It is another object of this invention to provide a flame type, flow reversal, regenerative exhaust afterburner in which the particular combination of passageways and combustion chamber offers minimum back pressure to the internal combustion engine feeding its exhaust gases thereto.

It is another object of this invention to provide an improved flame type regenerative exhaust afterburner ineluding a combustion chamber, a pair of spaced regenerative heat exchangers at each end of the combustion cha1nher, a conduit extending longitudinally of the combustion chamber and communicating at one end with an adjacent end of said chamber.

It is another object of this invention to provide a manifold assembly for anafterburner of the class just described having four chambers, valve means movable between alternate pairs of said chambers for achieving direct flow and reverse flow, and valve operating means for controlling valve movements.

It is another object of this invention to provide a manifold assembly fora flame. type, flow reversal, regenerative exhaust afterburner includinga housing, valve means, valve operating means, and control means for controlling and periodically reversingthe flow of exhaust gases" through thecombustion chamber of an afterburner. It is another object of this invention to. provide 'a combination ofafterburner and manifold therefor which coact to produce an afterburner system for internal combustion engines which not only very efiiciently cleans up exhaust gases but also offers very low back pressure to the associated internal combustion engine.

' The invention is further illustrated by reference to the attached drawings wherein:

FIG. 1 is atop plan view of an embodiment of a regenerative automobile exhaust afterburner, some parts thereof being broken away and some parts being shown in section;

FIG. 2 is a side elevationalview of the embodiment shown in FIG. 1, some parts thereof being broken away and some parts being shownin section;

FIG. 3 is a view in vertical section taken on the line 3-3 of FIG. 1, some parts vbeing shown in elevation;

FIG. 4 is a view in elevation as seen from right to left of. FIG. 1, sjomeparts thereof being brokenaway and some parts being shown in section; a

FIG. 5 is an enlarged view in longitudinal sect-ion taken along the line 55 of FIG. 2, some parts being-broken away and some parts being shown in section; 1

FIG. 6 is an enlarged view in longitudinal sectio taken along the line 66 of FIG. 2, some parts thereof being broken away and some parts being shown in section;

FIG. 7 is a further enlarged detailed view in vertical section taken along the line 7-7 of FIG. 1;

FIG. 8 is a further enlarged detailed view in vertical section taken along the line 8-8 of FIG. 1;

FIG. 9 is an enlarged elevational end viewin detail similar to FIG. 2 showing a valve operating mechanism for the structure shown in FIGS. 1-9; and

FIG. 10 isa fragmentary view in longitudinal section of a modification of the embodiment shown in FIGS. 1-9.

Turning to the drawings there is seen in FIG. 1 an embodiment of a flame type, reverse flow, regenerative exhaust afterburner, herein designated in its entirety by the numeral 20. This afterburner 20 is seen to contain a combustion chamber 22 within a housing 21. A pair of longitudinally spaced regenerative heat exchangers 23 and 24 are positioned one in each end portion of the com-bustion chamber 22.

As those familiar with afterburner systems for internal combustion engines will appreciate, a flame type afterburner operates to remove the combustibles remaining in the engine exhaust gas by oxidizing in a high temperature fiame reaction the hydrocarbons, carbon monoxide, and hydrogen in such exhaust gas. The dimensions and design considerations of a direct flame type afterburner of this invention must take into account a number of variables associated with the particular type of internal combustion engine with which the flame type afterburner is to be used. Such variables include concentration of combustibles in the exhaust gas, percent excess oxygen present, volume of the reactor chamber, pressure in the reactor chamber, specific reaction rate, reaction temperature, and the like.

In general, to maximize the rate of oxidation of combustibles in the exhaust gas, the highest practicable re action temperature consistent with efiicient operation, materials of construction, etc., must be maintained at all times in combustion chamber 22. Also, the percent reduction in concentration of combustibles (termed herein clean-up) achieved in an afterburner is higher for a reaction at a fixed temperature the higher the concen-' tration of combustibles is within the-reaction chamberg.

Experience seems to indicate that when one isu'sing afterburner 20 with a conventional four-stroke spark-ignition engine operating at atmospheric conditions on a premixed air-fuel mixture that reaction temperatures in the range offrom about 1200 F. to 2200 F. are suitable in cornbustion chamber 22 using presently available materials of construction.

, Although a large volume for combustion chamber 22 provides a greater gas residence time, presumably leading to greater reaction completion at'a specific temperature, still the larger the chamber 22 is, the more the heat loss through housing 21, and, consequently, the lower the reaction temperature for a given type of construction. As a compromise value combustion chamber 22 can have a volume of from about 250 to 350 cu. in. of a premixed air-fuel mixture when afterburner 20 isbeing used to burn exhaust from a conventional four-stroke sparkignition engine operating at atmospheric conditions.

The range of clean-up realized during residence time of exhaust gases in combustion chamber 22 is very great owing primarily to the enormous changes in the levels of combustibles and contaminants in the input exhaust gas stream. Such changes in input exhaust gas stream composition come about because of the ever changing modes of engine operation. Typical values reported for automobile engines in the literature show the carbon monoxide concentration varying from about 0.3% to 11.0%, the hydrogen concentration varying from about 0.1% to 5.0%, and the hydrocarbon concentration varying from about to 9500 parts per million (ppm) equivalent hexane. The range of these figures necessitates compromises in the design of the afterburner. Currently, for

cient air rnust be added to :produce a stoichiometricfrnixpipe '69 isconnected toafterburnerfltl (see, for example,

wall 73' are so "ehosen' thattogether they can emused- I hou'singf21 'and heat exchangers 23 and; 24, and be v tween the housing 21 andthe refractory wall 73, is placed; V afliner'74which is usually of a;therniallystable;insulat- 70" example, the State of California has set the standard'sithatj the exhaust gas discharge from a vehicle'shall not contain 7: i more than about 275 ppm. HC and 1.5% 'CO. For these cease. V e I c In order to cause combustion ofenglne exhaust gases a in combustion chamber 22, supplementary air must be added toiexhaust gases issuing from aninternal' com A, bustion engine, such asja conventional automotive engine, 1 before or at the time such exhaust gases enter the afterburner 20. The maximurnlamount of supplementary-air that'ean be added for oxidation is limitedbecaus'eexcessive quantities of supplementaryfair have"undesirable side effects. Thus,excess supplernentary air i (a).tends to quench the exhaust gas str eam from an engine thereby 1 undesirably lowering-its temperature, (b) tends to increase the flow rate through'theafterburner thereby decreasing residencetime in the combustion chamber 22, and. I (0) tends to dilute the'combustibles in the exhaust gas 7 thereby [decreasing the overall concentration of combustibles entering combustion chamber;22. e s V The amount of supplementaryrair is controlled Withinv 'limitsdi'rectly relatedto the concentration of combustibles entering combustion'rchamber 22 from an engine; Sufii tureoficombustibles in the'exhaust, gas andoxygen -in' the air with preferably a small excess of oxygen. i It'has'been r. found-that for four-strokespark-ignition engines operating on a premixed air-ffuel mixture the amount of "supple i mentary air-' added can'va ry from betWe'enS -to 1O% ofthe original 'gasstream issuefrom an engine with the averagei quantity of supplemental air added being about:

. -7.-5%-;of"'the' average gas flowlj Thus; more excess,air-

1 is added atflow exhaust flow rates producing the desirable eflFect of"having a higher concentration 0? cornbustibles at'such time. 3 g 1 f; L Inafterburner 20 supplementary air-is 'addedtolengine exhaust pipe 69-:just,prior to. the point where exhaust FIG. 1). yThe .air' isconveniently passed into the pipe 69 from a forced air feedpipe 7t) which is forked at its 7 end into a pair of delivery pipes71and72 which bleed air'into engine-exhaust pipe 69 from'foppositel sides. Any convenient blower. arrangement can be used' as a, means fOr forcingfiair throughf pipe 70 at the desiredrater Since such a blower, arrangementdoes not constitute a 1 part "of this"in'vention arid its operation and constructionl is conventional, no further description of the means for supplying reread air to the reed pipe" is givenjherei' at the time; of fabrication. The dimensions of the spectiveheat exchangers :23' and 24 and of tlie refractory withinihou'sin'g '21 as a singl'e uniti Between the wall 7 of ing, resilient material such as asbestos fwhich makes a a snug fit between heat exchangers 23;arirl l t,jefractoryi j wall 7 3 and 'hQ'using 21, darnpens-fvibrationa1';shocls to afterburner 20, and helps insulate afterburner; 20 against thermal shocks; Securing'the assembly cinnjp 'sing the methods of fastening can be'employed.

direct and ch annelexhaust. through-"afterburner201;; Referring now 'in greater detail to manifold-29,' i't is' seenthat this. 'structur elha's an inlet heat exchangers 23,24, refractoryrwall 73 and liner 74,

in place within housing 21, are, at oppositeends of the housing 21, header 281 and jmanifold 29, whose respective ,functions will be described hereinafter, and which so engage housing 21 and respective heat exchangers 24 and 23 as to hold all parts, in fixed relationship to one, another, Between jheader. 28 and heat exchanger 24 is' a spacer and between manifold-29and heatjexohanger 23. is a spacer 76;;these spacers"75 and 76are usually. of'

semi-rigid resilient eonstruction; and are made vfrom a heat-resistant or even refraetory material. I

Positioned adjacent the outside of housing 2l andextending 'generally:longitudinally thereof and parallel to V V eac h othenii's a pair of conduits-26 and 27. Conduit 27 15 is opento the atmosphere at one end and joins manifold 29 at: its other end; 'Co'nduit 26 at one end joins header 28 and at the other-end joins manifold 29; Secured to the outer side "ed gesfrespec tively, of header 28 and mani f old 29, areit-he opposite side edgesof a casing 31 which 'circurnscribe's"'housingj 21 aridconduits 26iand' 2 7," Casing 31;?is an optional featu're' of the embodiment-shown in FIGS 1 through 10, and serves' the purposes of'protectinghousing 21-,"conduit 26 andconduit27 from road dirt; acting iasaheaf shield retarding escape of heat .5 A e c In general, header 28' and manifold '29 can beirnade from cast'-lieat-resistantJrnetal; housing21' andflca'sin'g 31 are made'of heat'resis'tant 'sheet'rnetalgand' C011duitS'26 and 2 7 are heat-resistant metal tubing. ,For purpose's ofr confromj combustion chamber 22;1 and 'irnproving silencing.

venience'in massproduction; the various" points of jointure are usually. welded together, although, as those skilled in the art willf appreciate, welding, screwing and other I Header281: serves1to join'combustion C anemaeludi the regioniiateither 1 end thereof comprised byjrespective'heatexchangers 23' and 24) withconduit Header 28 is of simple construction andi'rnerely defines the desired connecting passageway 39a between chamber 22 and 'conduit"2 6'.'? 'ff' J 1 Manifold 29, "atthe oth'er end of housing -21, serves to gases from the'engine into and port 36', an exhaust port ej, ajreverseflow part38'and'a direct flow port 39. Direct, flow-j port 39 communicates 'ljfromthe housing r mei'nifoldg29 ta combustionfchainber 6'22. Reverse flow =port 38 communicates I fromthe' housing orrm'anitold 2 9F toV- conduit'26. Inlet por 351 com:

V r'nunicates' to the housing; of; manifold 29 from engine ex- 50; v o housingjof manifoldr 29 'ftoconduit 27'1" g The? terior of ill! manifold-22E divided into, four I interconnecting charn ersfi, A fir'st 'chambcr 141" com-, municates vvith inletportr36;-:ag second ichamber 421cm- "municates with reverse'flow port.3 8;la'fthird cha1nbfer 43 communicates withexhaust portj37;tiandiarfourth chaml be'rf44 communicates with directflowport 39.;- Butter- "Tbetween the chambers-41,"42, 12and44fthat valve46 may first be 'moved' to 'partit ion firstlcharnber '41 and fourth chamber-44' from second chamber- 42 l and third ch e 4 a dt h nre qu lyr aybe m v dv 'o as to V partition second. chamber; '42 :and first *cha'rnber 41 from the. desired movements to butte'rfly. valve 46,; afsh'aft' '47 the; center jOf butterfly valve' "46" so a Manifold 29. is equipped'with is; an M11 As which is I "conveniently ,demountably. attached thereto, 'as f by ma- "chine screws or sirnilar fastening means, which s it sockets 54.: Shaft141 is appropriately,vjournalled in end wall 48 sothat whenjend wall I 48. is1mounted, on manifold to a vertical position. tioned upon end wall 48 by the use of a solenoid mount- 5 29, the-valve 46 is permitted to rotate or oscillate in the desired manner to provide partitioning movements within manifold 29. Shaft 47 thus extends rearwardly of end wall 48. -Conveniently and preferably, the housing of manifold 29 is so constructed that the edges of valve 46 almost physically contact the respective partitioning 'walls of the individual chambers, so as to make as compartitioning chambers 42 and 43 from chambers 41 and 44 to a second position partitioning chambers 41 and 42 'from chambers 43 and 44. Although any conventional means can be used to move butterfly valve 46 from one partitioning position to the second, one simple means for accomplishing such movement of butterfly valve 46 is shown in FIG. 9.

Here, the a fterburner is equipped for use in an automobile having a conventional internal combustion engine and a l2-volt DJC. ignition system. The afterburner is mounted beneath the body of the car being suitably strapped or otherwise secured to the frame members of the car (not shown in the drawings). While the distance between inlet port 36 and engine manifold 29 is not critical, the distance is usually, as a practical matter, of the "order of five feet or so. A 1 2-volt DC. motor 56 is conveniently mounted in proximity to the afterburner, for example, against a vertical side of frame members beneath the floorboards of a conventional automobile. This moitoris chosen so as to have a constant speed of rotation of about lt) revolutions per minute or 1 revolution about every 6 seconds. To the shaft 57 of motor 56 is secured a timing cam 58. This cam is designed so that it causes the micro-switch 59 to close and thereby actuate a solenoid 61 for 180 of cam 58 rotation (i.e., .a total interval of time equaling 3 seconds) thereby moving the v-alve46 Observe that solenoid 61 is posiing plate 66. When the solenoid 6-1 is actuated, pulling armature-equipped push rod 62 downward (in the embodiment shown in FIG. 9) crank 63 is moved downward. Onefend of crank 63 is pivotally connected to armature-equipped push rod 62. The other end of crank 63 is rotatably connected to the free end of crank arm 65. The fixed end of crank arm 65 is rigidly connected to the exposed end of shaft 47. When push rod 62 moves When, after solenoid 6 1 is energized and armatureequipped push rod 6 2 has moved downward, valve 46 is moved to a vertical position. Thus, the valve 46 is moved *fI'O-ll'l DIw partitioning position to the other once every three seconds,

Heat exchangers 23 and 24 are of conventional design. The sensible heat of the exhaust gas input stream entering combustion chamber 22 must be significantly greater than that of theexhaust gas stream leaving the engine for all cruise conditions, in order to obtain combustion and the desired clean-up in exhaust chamber 22. The temperature of the oxidation reactions in the combustion chamber 22 is derived from the transformation of the latent or sensible heat of the gas stream to some equivalent temperature rise. Thus, for example, if a reaction temperature of "1750 F. is necessary to bring about a desired reduction -in combustibles and consequent desired percent clean-up,

and the temperature realized from the combustion reactronin combustion chamber 22 is 250 F., then the temperature or sensible heat of the gas stream entering the "reaction chamber must be 1500 F. The combustion reaction itself is thus seen to depend on the temperature of the gas entering the combustion chamber 22, the concentration of oxidizable reactants and the degreeof clean-up or decomposition desired. Actually, experience shows that a large fraction of the total exhaust gases issuing from a cruising automobile, for example, is relatively low in concentration of combustible contaminants. Consequently, one must in some manner add sensible heat to the exhaust gas input to the aftenburner because the combustion reactions are not self-supporting.

The necessary sensible heat is added by the use of heat exchangers 23 and 24, which are recuperative. That is, such heat exchangers can be first heated, then cooled and reheated again, cyclically, by alternately passing heated gases therethrough crosswise, first in one direction, and then passing cooler gases to be heated therethrough in the reverse direction. Suitable heat exchangers for use in the afterburners of this invention can be designed using any suitable configuration or construction. The interaction between the regenerative heat exchangers and combustion in the combustion chamber in an actual aflterburner is extremely difiicult to express or describe owing to the large number of parameters involve-d; Indeed, the combustion process and interrelationship between combustion chamber and heat exchangers is best understood by using specific operating modes. It follows that there are many variables which enter into the selection in any given instance of a particular heat exchanger .for any given regenerative flame-type af-terburner of this invention. The exact size, heat capacity, mass, etc., ofa heat exchanger for use in an embodiment of this invention must take into account the special circumstances of the individual case, in accordance with conventional engineering know-how. Actual selection of a heat exchanger for any given .afterburner system of the invention is a matter within the province of ordinary engineering skill, to be determined in accordance with the requirements of a particular use situation. Since the structure, construction and characteristics of the heat exchangers 23, 24, are conventional and do not form .a critical part of the present invention, no further description of thernis needed or given here.

A conventional glow-plug or spark plug 77 is provided for theinteri-or of combustion chain-ber 22 so as to provide initialignition means for gases to be oxidized in combustion chamber 22. i In the embodiment shotwnin )FIGS. 1 through 9, this glow-plug 77 is of conventional type and is mounted through. a hole extending through casing 31 (if present), housing 21, liner74and refractory wall 73. The glow-plug 77 is positioned in place by means of a mounting plate 7 8thr0ugh which the,glowplug extends normally. The mounting plate is secured in place by any conventional means,- for example, by bolts threading casing 3 1 and, if desired, also housing 2 1.

In the modified embodiment partially shown in FIG. It), a casing 32 not only serves as a shield against road d1rt but also serves as a passageway for exhaust gases leaving said manifold 29. Thus, instead of the exhaust gases leaving manifold 29 entering an exhaust conduit 27, they enter directly the passageway orplenum space 32 lying between housing 21 and casing 32.' In this modified embodiment, use is made of the exhaust gases to heat the exterior walls of housing 21 so as to conserve heat. A short pipe 34 is then provided to cbnductexhaust gases from a passageway 33 to the atmosphere.

I In general, over-capacity in the heat exchangercontributes to over-temperaturing' in the combustion chamber 22 at any given specific mode, thereby producing'excessive available thermal energy for heat transfer in the region of the combustion chamber 22. The overall efchanger, they could be completely misaligned so as to provide only a 36% open area.

The cross-sectional area of each conduit 26 and 27 is about 3 sq. in. The minimal cross-sectional area in the manifold in the chamber spaces between inlet port 36 and exhaust port 37, reverse flow port 38 and direct flow port 39 is generally never less than about 4 sq. in.

This afterburner is connected by means of an engine exhaust pipe to a conventional gasoline engine of the type used to power a typical American passenger car, specifically, a 283 cu. in. V-8 engine rated 160 HP. output at 4200 r.p.m. whose intake manifold vacuum at 50 m.p.h. is about 14.5 in. of mercury.

A steady state engine condition is selected, to wit, 25.8 HP. output at 2150 r.p.m. which corresponds to a typical passenger car cruising on a level road at 50 mph. To provide oxygen for combustion 8.5 c.f.m. of fresh air is added to the exhaust flow (roughly 37.5 c.f.m. at standard conditions) ahead of the afterburner. A total flow at standard conditions of 4-6 c.f.m. at exhaust gas tempera ture, 750 F. With valve 46 in horizontal position (see FIG. 7), flow is then through front heat exchanger 23. Heat is transferred from the heat exchanger 23 to the flow, whereupon the mixture burns in the combustion chamber 22 and reaches temperatures of 2200 F. The flow then continues through the rear heat exchanger 24 giving off heat increasing the temperature of heat exchanger 24 and decreasing the temperature of the flow to 1000 F. The flow then travels back through the conduit 26 and past valve 46 and to the atmosphere. After approximately 3 seconds in the horizontal position, the valve 46 is changed to the vertical position causing the flow to reverse itself and then repeating the process above. Three seconds has been chosen as the time interval since any longer period will result in the exchanger having insufiicient heat to provide high enough temperatures for combustion. initially, when the heat exchangers are cold, the combustion is started by the spark plug. Once started the spark is not necessary.

The pressure drop under the conditions described across the entire afterburner is 1.4 in. of mercury. Of this 0.9 in. of mercury occurs across the heat exchangers and combustion chamber. The untreated inlet hydrocarbon content was 375 p.p.m. and the CO content was 3.9%. After further combustion within the afterburner, a value of 80 p.p.m. hydrocarbon and 0.5% CO was achieved.

In summary, the present invention is directed to a regenerative afterburner for exhaust gases from internal combustion engines comprising elongated housing means defining a combustion chamber, a pair of longitudinally spaced regenerative heat exchangers each disposed in a different end portion of said chamber, conduit means extending longitudinally of said housing means and communicating at one end with an adjacent end of said chamber, a manifold included in said housing means at the other end of said chamber and having inlet port, exhaust port, and reverse flow port communicating with the other end of said conduit means, and a direct flow port communicating with said other end of said chamber, valve means in said manifold and including a valve element movable between direct flow and reverse flow positions, said valve element in its direct flow position effecting communication between said exhaust and reverse flow ports, said valve element in its reverse flow position effecting communication between said inlet and reverse flow ports and between said exhaust and direct flow ports, valve operating means for shifting said valve element be tween said positions thereof, and gas combustion initiator means in said chamber.

The invention also relates to a manifold for a flame type regenerative afterburner, said afterburner including elongated housing means defining a combustion chamber and conduit means extending longitudinally of said housing means and communicating at one end with one end of said combustion chamber, said manifold comprising a manifold housing defining an inlet port for receiving exhaust gas input, an exhaust port communicating with the atmosphere, a reverse flow port communicating with one end of said conduit means, and a direct flow port communicating with the other end of said afterburner housing, said manifold housing further defining four interconnecting chambers, a first of which communicates with said inlet port, a second of which communicates with said direct flow port, a third of which communicates with said reverse flow port, and a fourth of which communicates with said exhaust port, valve means in said manifold and including a valve element shiftable between spaced operative positions, said valve element in one of said positions establishing communication between said first and third chambers and between said second and fourth chambers and in the other of said positions establishing communication between said first and second chambers and between said third and fourth chambers, valve operating means for shifting said valve element between said operative positions thereof, and control means periodically rendering said valve operating means operative to shift said valve.

Although the foregoing description has been made with reference to certain preferred embodiments of the invention, it will be appreciated that the afterburner system and, in particular, the exhause afterburner itself and the manifold therefor, can be used with substantially any internal combustion engine. Thus, the invention is not restricted to use with conventional four-stroke spark-ignition engines operating at atmospheric conditions, or conventional fuels therefor, but can be used with any internal combustion engine using vaporizable or air-dispersable, oxidizable fuel. More broadly, the afterburner and manifold of this invention can be used in any situation where one desires to oxidize a combustible fuel mixture in order to avoid releasing such mixture to the environment. The references in the specification to operating conditions and design considerations for the afterburner system of this invention are for purposes of illustration only, and do not constitute any limitation upon the inventive subject mater of this application as set forth in the appended claim.

It is to be understood that this invention is capable of modification without departure from the spirit and scope of such appended claim.

I claim:

A regenerative afterburner for exhaust gases from an internal combustion engine comprising an elongated housing, a pair of regenerative heat exchangers fixedly mounted in longitudinally spaced relation in the housing, a manifold having an inlet mounted on one end of the housing engaging one of said pair of heat exchangers, a header mounted on the opposite end of the housing engaging the other of said pair of heat exchangers, and a spacer ring of refractory material between and in engagement with said heat exchangers, valve means in the manifold, conduit means connecting the header and manifold, and means for periodically shifting said valve means to alternately connect the inlet of the manifold to one end of the first heat exchanger and connect one end of the conduit means to exhaust and to then connect said one end of the first heat exchanger to exhaust and the conduit to the intake whereby to provide for a straight line fiow of gases through the heat exchangers first in one direction and then in the opposite direction.

References Cited by the Examiner UNITED STATES PATENTS 2,898,202 8/59 Houdry et al. 3O X 2,937,490 5/60 Calvert 60-30 3,086,353 4/63 Ridgway 603O JULIUS E. WEST, Primary Examiner. 

